<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>结果：</div>
</body>

<script>
  let i1 = document.querySelector("#i1")
  let i2 = document.querySelector("#i2")
  let d = document.querySelector("div")

  function f(x) {
    if (isNaN(i1.value) || isNaN(i2.value)){
      d.innerText = "输入有误！！"
      return;
    }
    switch (x) {
        case 1:
            d.innerText = "结果："+(parseFloat(i1.value)+parseFloat(i2.value))
            break
        case 2:
            d.innerText = "结果："+(i1.value-i2.value)
            break
        case 3:
            d.innerText = "结果："+(i1.value*i2.value)
            break
        case 4:
            d.innerText = "结果："+(i1.value/i2.value)
            break
    }
  }
</script>

</html>